Re: [PATCH v4 08/23] ewah: compressed bitmap implementation

On Thu, Jan 23, 2014 at 09:53:26PM +0000, brian m. carlson wrote:

On Thu, Jan 23, 2014 at 03:26:45PM -0500, Jeff King wrote:

quoted

Looking over the format, I think the only thing preventing 4-byte
alignment is the 1-byte XOR-offset and 1-byte flags field for each
bitmap. If we ever have a v2, we could pad the sum of those out to 4
bytes. Is 4-byte alignment enough? We do treat the actual data as 64-bit
integers. I wonder if that would have problems on Sparc64, for example.

Yes, it will.  SPARC requires all loads be naturally aligned (4-byte to
an address that's a multiple of 4, 8-byte to a multiple of 8, and so
on).  In general, architectures that do not support unaligned access
require natural alignment for all quantities.

In that case, I think we cannot even blame Shawn. The ewah serialization
format itself (which JGit inherited from the javaewah library) has 8
bytes of header and 4 bytes of trailer. So packed serialized ewahs
wouldn't be 8-byte aligned (though of course he could have added his own
padding to each when we have a sequence of them).

-Peff